Strain in GaAs quantum wells and layered composites detected by optically pumped NMR(Conference Presentation)

We present a methodology for characterizing lattice strain effects in crystalline semiconductors based on optically pumped NMR (OPNMR). Lattice strain is detected as an electric quadrupole splitting of the NMR transition. Since OPNMR is an optical technique, it selectively probes strain only in the volume within the optical penetration depth of the laser light. The methodology is demonstrated in (1) variably thinned bulk GaAs layered composites and (2) GaAs quantum well thin films. Thermally induced lattice strain was induced by epoxy-bonding to Si support wafers at 373 K followed by cooling to 1.5 K. The variation of the strain with GaAs layer thickness is shown to be consistent with an analytical model for mechanical bowing. In the GaAs/AlxGa1-xAs thin films, the strain measured from the quadrupole splitting of the 71Ga NMR transition was incorporated into electronic energy band structure calculations which yield the photon energy dependence of the optical absorption and conduction electron spin polarization. The nuclear spin polarization is calculated from the electron spin polarization using an appropriate electron-nuclear cross-relaxation model. Comparison of theory to the experimental data provides new insights into how the optically pumped nuclear spin polarization is affected by strain and quantum confinement. [1] M. Sturge, Phys. Rev. 127, 768 (1962)[2] Y. Sun, et. al., Strain Effects in Semiconductors: Theory and Device Applications (Springer, 2010).[3] P.L. Kuhns et al., Phys. Rev. B. 55, 7824-7830 (1997).[4] R.M. Wood et al., Phys. Rev. B. 90, 155317 (2014)